The invention relates to an actuator with an electrically heatable thermostatic operating element heated by an electrical heating element and having a housing containing an expanding material and an operating piston that is extensible from the housing, the piston position being controlled by a proportional piston stroke regulator.
An actuator of this type is known, for example, in German Patent Disclosure DE 41 38 523 A1. In that construction, the regulation of the position of the operating piston is based on the electrical resistance of the thermostatic operating element, which is assumed to vary as a function of the change in volume of the expanding material, so that the electrical resistance of the operating element corresponds to the position of the operating piston. This design is not totally reliable as changes in ambient temperatures, ambient pressures or combinations thereof potentially change the expansion for a given temperature as measured by the operating piston position since the electrical resistance measured is not truly proportional to the actual temperature. Furthermore, the starting ambient temperature surrounding the operating element would either have to be a constant, so the piston always started at the same position with the same corresponding measured electrical resistance, or the piston position would have to be re-calibrated before every use of the operating element. The operability of the exemplary embodiments with which an electrical resistance varying with the change in volume of the expanding material is to be detected is not always consistent with precise and accurate movement and regulation of the operating piston.
It is the object of the present invention to provide an actuator with a way of precisely measuring and regulating the position of the operating piston.
This object is attained by controlling the operating piston position with a proportional piston stroke regulator, which includes a piston travel detection device connected to a conventional closed-loop control circuit. The closed-loop control circuit compares the detected position of the operating piston with an entered pre-determined piston control position and regulates the electrical current supply to the electrical heating element accordingly.
By the present invention, it is possible to provide very precise measurement and regulation of the operating piston position. In particular, very precise movement of the operating piston to pre-determined positions is possible.
In a specific embodiment of the invention, the conventional closed-loop control circuit controls the supply of pulses of alternating current to the electrical heating element by the use of a relay, in particular a triac. By supplying the electrical heating element with an adjustable intermittent duration of pulses of electrical current, the electrically heatable thermostatic operating element behaves in the manner consistent with a proportional, integral, derivative (PID) controller, enabling very precise piston position regulation. Precision is enhanced if the operating temperature of the expanding material, that is, the temperature at which the expanding material begins to change from its original unheated state, is markedly above the ambient temperature, so that relatively rapid cooling and retraction of the operating piston occurs when the electrical heating element is not supplied with electrical current.
In another specific embodiment of the invention, the maximum travel of an actuator element of a device to be actuated is determined and stored in the memory of the conventional closed-loop control circuit. It is thus possible within the closed-loop control circuit to detect the maximum travel of the device to be actuated, for example a tappet of a valve, and to evenly distribute the pre-determined piston control positions along the entire length of travel, for example between the opening position and the closing position of a tappet of a valve, thereby providing a number of adjustable positions available to the actuating element of the device to be actuated.
In a specific structural embodiment of the invention, an outer housing encloses a stationary base body on which the electrically heatable thermostatic operating element, a motion transfer member moveable with the operating piston and the piston travel detection device are supported, the piston travel detection device being disposed between the motion transfer member and the outer housing or the base body. A preferred construction of the piston travel detection device includes a stationary element, mounted on the outer housing or the stationary base body, and a relative position element associated with the stationary element and moveable with the motion transfer device and the operating piston.
In a specific feature of the structural embodiment of the invention, the relative position element moveable with the motion transfer device and the operating piston is disposed on a circuit board which is mounted on the motion transfer member. It is advantageous if the conventional closed-loop control circuit and the relay are disposed on the circuit board as well.
Further characteristics and advantages of the invention will become apparent from the following description of the exemplary embodiment shown in the drawing.